Polyolefins are a class of polymers derived from simple olefins. Known methods of making polyolefins involve the use of Ziegler-Natta polymerization catalysts. These catalysts polymerize vinyl monomers using a transition metal halide to provide a polymer with an isotactic stereochemical configuration.
One type of Ziegler-Natta catalyst system comprises a solid catalyst component, constituted by a magnesium dihalide on which are supported a titanium compound and an internal electron donor compound. In order to maintain high selectivity for an isotactic polymer product, a variety of internal electron donor compounds must be added during catalyst synthesis. Conventionally, when a higher crystallinity of the polymer is required, an external donor compound is also added during the polymerization reaction.
During the past 30 years, numerous supported Ziegler-Natta catalysts have been developed which afford a much higher activity in olefin polymerization reactions and much higher content of crystalline isotactic fractions in the polymers they produce. With the development of internal and external electron donor compounds, polyolefin catalyst systems are continuously renovated.
U.S. Pat. Nos. 4,784,983 and 4,861,847 describe a catalyst system for use in olefinic polymerization and copolymerization that is comprised of components (A), (B) and (C). The catalyst component (A) consisting essentially of titanium, magnesium, halogen, polycarboxylic acid esters and organic phosphorus compounds is a solid product, being prepared by mixing titanium tetrahalide and auxiliary precipitant with a homogeneous solution of magnesium halide in a solvent system consisting essentially of an organic epoxy compound and an organic phosphorus compound to form a solid product which is then treated with a polycarboxylic acid ester and titanium tetrahalide. Component (B) is an organic aluminum compound, and component (C) is an organic silicon compound. The catalyst system has a very high activity, and the resultant polymers have very high stereospecificity and good granular appearance.
U.S. Pat. No. 6,376,417 describes a catalyst for the polymerization of propylene comprising components (A), (B) and (C). Component (A) is a solid product prepared by dissolving a halide of magnesium in a solvent system consisting of an organic epoxy compound, an organic phosphorus compound and an inert diluent to form a homogeneous solution; mixing the homogeneous solution with a halide of titanium to form a mixture; precipitating a solid from the mixture in the presence of an auxiliary precipitant; treating the solid with a polycarboxylic ester to load the ester on the solid; and treating the ester-loaded solid with the halide of titanium and the inert diluent. Component (B) is an organic aluminum compound, and component (C) is an organic silicon compound. The particle size of the catalyst can be adjusted by increasing the amount of the inert diluent at a low ratio of the epoxy compound to the phosphorus compound. However, in some cases, increasing the particle size by increasing the amount of inert diluent results in irregular catalyst morphology, for example, broadening particle size distribution, particle elongation, or reduction of bulk density. In addition, the increasing amount of the inert diluent to produce a larger particle size catalyst can be limited by the reactor size in a production scale.
The general production scheme for MgCl2 supported catalysts includes a process to make MgCl2 support, impregnation of TiCl4 and internal donor to the MgCl2 surface and the catalyst activation. One of the methods of MgCl2 supported catalyst preparation is dissolution of solid MgCl2 with organic reagents and precipitation of MgCl2 with certain morphology. Catalyst morphology control is one of the most important aspects of industrial polyolefin plant operation. Catalyst morphology characteristics include particle size and particle size distribution, particle shape, bulk density, and stickiness. Catalyst morphology characteristics influence plant operation efficiency, polymerization kinetics and final polymer morphology. For example, unsuitable catalyst morphology may cause failure in polymer morphology control, which can lead to serious trouble in plant operation, such as fouling or sheeting.
Because of these reasons, MgCl2 supported catalysts with good morphology control (required particle size and shape, narrow particle size distribution, high bulk density and low stickiness) are desired.